1. Field of the Invention
The presently disclosed invention relates to rooftop mounting systems for photovoltaic modules and, more particularly, mounting systems that serve as a roof covering.
2. Discussion of the Prior Art
A variety of mounting systems for photovoltaic modules are known in the prior art. In some cases, photovoltaic modules or “PV modules” serve as commercial power stations for generating commercial levels of electric power. In those power station systems, ground-level mountings systems support large numbers of photovoltaic modules that are arranged in a two dimensional array that covers a substantial area, often measured in units of acres. The mounting systems are relatively sophisticated. For example, they often control the orientation of the solar modules in two axes so that the modules track the position of the sun throughout the day in order to more effectively capture the solar energy.
Other, somewhat smaller systems are used in commercial or government installations. These systems are often dedicated to a particular commercial or governmental building or group of buildings such as a manufacturing facility or school. In many cases, such commercial systems are installed on rooftops that, due to the size of the building, are flat roofs. In some cases the mounting systems will cause the photovoltaic modules to track the position of the sun similar to power plant systems. More frequently, the commercial mounting systems hold the photovoltaic modules in a fixed angle and direction that is calculated to improve the efficiency of the modules in comparison to the achievable efficiency of a horizontal orientation. Such systems include the SunLink® RMS which is commercially available from SunLink Corporation of San Rafael, Calif.; and the SunPower® T5 Solar Roof Tile which is commercially available from SunPower Corporation.
In terms of the number of installations, residential applications are the most common use for photovoltaic modules that are capable of generating electrical power at levels suitable for household use. For reasons of user safety, product efficiency, protection of the power modules, and availability of space, photovoltaic modules that are used in residential applications are often mounted on the roof of the residence. Most residences are constructed with a peaked roof having a slope of as much as three to one. The mounting systems that have been developed for power generation stations and commercial installations are typically intended for horizontal surfaces and often are not directly applicable to sloped roofs. Moreover, those mounting systems are too massive and expensive for practical application on a residential, sloped roof.
Residential photovoltaic power systems generally incorporate the use of PV modules in the size of about three feet by five feet by two inches. The PV modules are comprised of a two dimensional array of crystalline photovoltaic cells or “PV cells,” The PV cells do not produce sufficient power to make in commercially feasible to sell and install them as individual cells. However, when the PV cells are organized in sufficiently large arrays and electrically connected together in series and parallel circuits, they produce power levels that are suitable for use in many applications. The practical and economic considerations for manufacturing a suitable array of PV cells generally require an array of approximately five feet by three feet, depending on the efficiency of the PV cells that compose the array. Although some arrays are much bigger, particularly for use in power generation stations and for commercial applications, the PV modules that are used in residential applications are generally kept to the smallest commercially achievable size. That is because the smaller size favors a more flexible application of the PV module. By using smaller PV modules, the PV modules and arrays of PV modules can meet a broader range of limitations on available space and cost such as is typically encountered in residential applications.
Like power station PV systems and commercial PV systems, residential PV systems also have mounting systems that are known in the prior art. One commercial example of such designs is the SunTile™ PV roofing system by PowerLight Corporation. However, residential PV systems that were known in the prior art have had various disadvantages.
Prior residential mounting systems for PV modules generally fall into one of several categories. In one category, the PV module was simply supported directly on the conventional roof cover. It was known in the prior art that PV modules produce heat and so this type of roof mount typically included a thermal insulation barrier between the PV module and the roof cover so that the heat would not invade the house. However, it was also known in the prior art that PV modules become less efficient as their operating temperature increases. Therefore, it was seen that mounting systems that could better dissipate heat away from the PV module would be preferable.
In another category, the mounting system holds the PV modules apart from the roof cover so that air flow over both the front and back of the PV panel better dissipates thermal energy and tends to limit the temperature of the PV modules. Mounting systems in this category have a set of feet that are supported on top of a conventional roof covering such as asphalt or wood shingles. In those systems, the feet set atop the roof covering and support a framework of a size and geometry that corresponds to the desired size and shape of the array. The PV modules are secured to the frame to complete the array. An example of such systems is the Smart Mount™ system which is commercially available from SunPower Corporation.
Another category of residential mounting systems for PV modules also is built over a conventional roof cover such as a tile, asphalt, slate or other roofing material and also has a plurality of feet that support a framework that holds the array away from the surface of the roof cover. However, in this category the feet are not supported by the roof covering. Rather, they penetrate through the roof covering and are secured directly to the support frame of the roof. Mounting systems of this type tend to be more secure, but they also have a disadvantage in that they result in a number of penetrations through the roof covering. That makes them more difficult and expensive to install and also creates higher risk that water will penetrate the roof cover.
In another type of residential mounting design, the PV modules were arranged in columns that were oriented with the slope of the roof. Each PV module in the column partially overlapped the next lower module. This arrangement was thought to be advantageous in diverting water in the way of asphalt shingles. However, it was also disadvantageous in that a portion of each PV module was shielded from solar illumination so that the efficiency of the PV module array was materially compromised.
Still another type of PV mounting system is known as the PV shingle. Strictly speaking, this is not a mounting system but, rather, a modification of the PV device itself so that it can be applied somewhat differently than PV modules that are composed of crystalline PV cells. In a typical example of this system, the PV power source is constructed of amorphous silicon PV cells that are included in a relatively thin, physically flexible sheet that can be shaped in the general form of an asphalt shingle. Such “PV shingles” are nailed to the roof deck in an overlapping fashion similar to asphalt shingles. PV shingles are considered by some to be more aesthetically pleasing, but they also lack air flow to the underside of the PV shingle. The absence of air flow results in relatively high operating temperatures for the PV shingle. The higher temperatures tend to decrease efficiency of the PV shingles and to increase heat transfer from the roof into the house. In addition, PV shingles have an inherently lower efficiency than crystalline PV cells so that a physically larger array of PV shingles is required to generate a comparable amount of power.
It was observed in the prior art that a mounting system for PV modules that could serve the dual function of supporting the PV module array and also operating as a roof cover would be advantageous. Such a mounting system would avoid the disadvantages of PV shingles and also avoid the disadvantages of mounting systems that either stand on existing roof coverings or that are connected through the roof covering to the roof support frame. Such mounting systems would enjoy the greater efficiency of PV modules composed of crystalline PV cells and would also be securely fastened to the frame of the residence while avoiding additional maintenance and risk associated with multiple penetrations of the roof cover. However, combination roofing tile/PV module mounting systems known in the prior art had various disadvantages and difficulties.
As explained previously, it is desirable to keep the physical size of the PV module small due to reasons of cost and design flexibility. However, manufacturing constraints require a generally minimum size for the PV module. Thus, to avoid further compromise in design and cost flexibility, it is desirable that the unit size for a dual function roof cover/PV module mounting system be no larger than the PV module so that a one-to-one ratio is maintained. However, some prior art designs in which the PV module mounting system also served as a roof cover failed to keep a one-to-one ratio between the PV modules and the unit size of the mounting system. Such designs included roofing tile that accommodated multiple PV modules so as to compromise flexibility in the application of the PV module mounting system. In other cases, the tiles were smaller than the PV modules resulting in a corresponding multiplication of parts for the system and additional failure points for the roof cover.
Other styles of combination roof cover/PV module mounting systems provided roofing tiles that corresponded to the PV modules on a one-to-one basis, but they created a border around the perimeter of each PV module. Thus, when the roofing tiles were joined together, the PV modules in the array were separated laterally from each other. Solar illumination of that portion of the array composed of the exposed portions of the tiles did not contribute to any power generation. In a somewhat converse design, other roof cover/PV module mounting systems wherein roofing tiles corresponded to the PV modules on a one-to-one basis provided an air gap between the PV modules. There also, solar illumination of the array was partially lost to the gaps between the PV modules and did not contribute to any power generation. Accordingly, a PV module array of a given size with either design of roof cover/mounting system was less efficient than a PV module array of the equivalent size wherein the PV modules were fitted closely together to form a planar illumination surface.
Still other styles of PV module mounting designs have provided a roof tile that supports a PV module on a one-to-one ratio with the PV panels fitted closely together. However, those systems supported the PV module at periodic locations that did not admit to interlocking engagement of the tiles in a way that the PV modules were supported by the tile directly under the PV module and by adjacent tiles. This limitation against integrating the tiles created a risk of water leakage between the tiles and required heavier gauge tiles to achieve mechanical strength that would be comparable to a system with more sophisticated interlocking capability.